Really depends on the functional form in the code which specifies probability of parry/block/dodge

I find it pretty hard to imagine that they would simply have a linear probability function. In this case certain classes (like invokers using daggers, slowed shapeshifters, etc) should find it actually impossible (function > 1 being set to probability = 1) to get past defenses, but they often do get past defenses now and then. It's more likely that they have some sort of probit/logit type function so that stacking more and more 'defensiveness' will only make your probability of deflecting an attack close to 1 at the limit, not actually 1.

In this case your flourintine might raise probability of parry from 0.8 to 0.9 in a given case to give 0.1 chance of getting past defense, and you might have 0.6 parry and 0.5 block to give 0.2 chance of getting past defense. (BTW dual wield gives a parry bonus over single wield)

Or they might code it as an 'extra parry'. Note a recent question posed on Officials about Ward of Blades and flourintine. Daevryn said that Ward with non-swords gives a total overall parry chance which is lower than swords with flourintine without ward - that makes flourintine AT LEAST as good as a second attempt to parry, given that swords parry better than all other 1-hand weapons. We know that Ward gives a second parry check alongside the first.

Given that Daev's answer was pretty unequivocal and lacked any caveats about situations/circumstances, I think it's plausible that flourintine is really a second parry chance, and that 2 attempts to parry with dual swords > 2 attempts to parry with dual axes.

I think there was also another question about 25 str parry with flourintine vs 25 dex parry with flourintine, apparently they are the same according to Daev. The benefit the dex warrior has is the dodge plus the higher riposte chance.